Foam is generated as a byproduct in many systems which include a circulation of water or other liquid. However, the generation of foam in most cases is only a problem when the foam becomes excessive and gets into other parts of the system. For example, in a waste water treatment system, air may be bubbled up through a tank of waste water to oxidize certain contaminants and facilitate their separation from the water. If there is soap or other foaming agent impurities in the waste water, a substantial amount of foam may be generated on the water in the tank. However, the foam would not otherwise present a problem until it overflows the tank and causes contamination in other parts of the treatment plant.
In a paper mill, for a second example, there are a number of systems that produce foam as a byproduct. For instance, foam is generated in the brown stock washer, the screen room, the bleach plant, and in the effluent. In each of these systems, a liquid, which is typically water, is circulated through a part of the paper making machine and picks up some sediment or other materials. In the process, the liquid may form a thick foam layer from the other materials in the liquid in combination with the agitation and turbulence of the flow. While a limited thickness may be acceptable, an excessive foam layer may overflow into the machine and contaminate the paper. Thus, the over production of foam must be avoided in a paper mill.
Foam is also generated as a byproduct in lubricating systems for large scale machine tools wherein the lubricating liquid is continuously recirculated. The liquid may pick up sediment and other debris that may cause a generation of foam. Excessive foam may impair the operation of the machine and interfere with the operation.
Clearly, foam may cause problems in a variety of systems. Fortunately a number of chemical defoaming agents are available to control such problems. However, defoaming agents vary widely in chemical formulation, cost, and effectiveness.
To control the foam, a conventional practice is to introduce an unattended continuous stream of defoaming agent into the system at a rate which is certain to prevent excessive amounts of foam. However, the foam producing impurities do not enter the system at a constant rate. Therefore, significant quantities of defoaming agent are wasted.
U.S. Pat. No. 4,009,118 to Laiho discloses an apparatus for controlling foam in a paper making system wherein the apparatus draws foam from the top of a filtrate basin and sprays a recycled defoaming agent onto the foam. As the defoaming agent becomes diluted over time, more defoaming agent is added to the recycling system. Laiho, however, is not suitable for handling very large amounts of foam that may be developed in the above described systems. Laiho also uses the defoaming agent at a rate which is preset and is not necessarily responsive to the current conditions in the filtrate basin. This risks using more or less defoaming agent than necessary and makes for a less than fully efficient system.
Accordingly, it is an object of the present system to provide an efficient system for controlling foam and which avoids the disadvantages of the prior art constructions as noted above.
It is a more particular object of the present invention to provide an efficient system for controlling foam which monitors the thickness of the foam and provides effective use of defoaming agent to maintain the thickness of the foam within a predetermined range.